This study was designed to clinically evaluate the incidence of recurrent caries and structual defects associated with Class I and Class II dental amalgam restorations in conservative versus extended cavity preparations. Up till now most dental schools teach cavity preparation in conservative dentistry following the method suggested by G. V. Black on the basis of "Extension for Prevention". We do not know of any controlled study demonstrating the clinical significance of strict adherence to those principles. A current trend has developed questioning the necessity of extension and, instead, emphasis has been placed upon conservation of tooth structure in relation to minimal exposure of amalgam margins to masticatory stress, conservation of tooth structure, less operational trauma, increased productivity of dental personnel, etc. Students of the Dental, Medical and Nursing Schools of the Hebrew University and students of ORT were recruited as patients, both because they were at an age where caries prevalence is high and because they can be followed up for 4 or 5 years. They were screened clinically and bite wing radiographs were made. The population consisted of 224 pairs of Class I restorations and 221 pairs of Class II restorations. One member of each pair is the test restoration (conservative preparation) and the other the control (extended preparation). Both members were located in the same dental arch and consisted of restorations placed in incipient lesions. The results of this investigation at the end of the 3rd year, do not show significant differences in caries recurrence between test and control fillings. It is likely that more meaningful figures could be obtained in the 4th and 5th year, thus authorizing an educated decision on whether to recommend one of the experimental preparations.